fix(ui): a read-ahead waits out a background refetch, and a revealed row is mounted before it is focused

NEXT_PAGE joins ANY fetch in flight: a read-ahead during an invalidation
or a poll joined that refetch and appended no page, and nothing asked
again until the reader moved the range (the channel page during a
backfill). The read-ahead now waits while the list fetches anything
(mayReadAhead, re-run on isFetching); a Load older click cancels a poll
instead (nextPageOnClick).

A page turn to a revealed row restarted the progressive mount at 30
rows; Channels focuses that row in the next commit, and at index 40 of
a 50-row page it was not there, so focus fell to <body>. The mount now
reaches through the revealed row.

frontend/src/lib/nextPage.ts

@@ -5,5 +5,21 @@
  * re-runs on every scroll frame, and a frame that lands before `isFetchingNextPage` turns true calls
  * again. On prod (E31 S4e) every feed page went out 2–3 times; on a 0.8 s title sort the copies
  * queued to 2.3 s. With this a second call joins the fetch already running.
+ *
+ * The join takes ANY fetch in flight, not only a next page: a call during a background refetch (an
+ * invalidation, a poll) joins THAT, which reloads the pages already there and appends none (E31 S5).
+ * So a read-ahead also waits while the list is fetching at all (`mayReadAhead`) and runs again when
+ * the fetch settles; a click uses `nextPageOnClick`.
  */
 export const NEXT_PAGE = { cancelRefetch: false } as const;
+
+/** Whether an infinite list's read-ahead may ask for the next page now. Not while ANY fetch of the
+ *  list is in flight: that call would join it (see NEXT_PAGE). The caller re-runs on `isFetching`. */
+export const mayReadAhead = (hasNextPage: boolean, isFetching: boolean) =>
+  hasNextPage && !isFetching;
+
+/** The options for a next page a person asked for (a "Load older" button): join the page already
+ *  loading, else cancel a background refetch and load the page — a click must not be swallowed by a
+ *  poll that happened to be running. */
+export const nextPageOnClick = (isFetchingNextPage: boolean) =>
+  isFetchingNextPage ? NEXT_PAGE : ({ cancelRefetch: true } as const);